A breakerless ignition system for an internal combustion engine is well known which comprises a semiconductor switching element, such as a thyristor disposed in a primary circuit of an ignition coil for controlling a primary current through the ignition coil by opening or closing the switching element in time with ignition point of the engine. A capacitor discharge type ignition system is typically employed. Such ignition system is provided with a signal source supplying a control signal to the semiconductor switching element in synchronism with rotation of the engine. In general, the signal source comprises a signal coil or coils disposed within a magneto generator in cooperation with magnetic field thereof. However, due to various coils, such as generating coil or coils for charging a capacitor in the capacitor discharge type ignition system and lighting coil or coils disposed within the magneto generator, there is no room in the magneto generator for mounting the signal coil or coils of the signal source. In particular, the breakerless ignition system for a multicylinder internal combustion engine requires a plurality of signals, which causes the signal coil design to be complicated, resulting in that assembly of the flywheel magneto generator is troublesome. In addition, the signal coil or coils disposed within the magneto tend to generate excessive signals as well as control signal during one complete revolution of the magneto generator because the number of the signals is determined by the number of the magnetic poles in the magneto generator. Such excessive signals must be removed out of the ignition circuit.
I have proposed a signal source for use in a breakerless ignition system for an internal combustion engine wherein a signal coil or coils can be mounted on a magneto generator without any interference. This has been disclosed in U.S. patent application Ser. No. 428,716, now U.S. Pat. No. 3,903,863 filed by the same inventor and assigned to the same assignee as those of this application. The signal source disclosed therein comprises a flush bolt securing one of magnets and associated pole piece to a bowl-like flywheel and extending through a hole in the bowl-like flywheel in a magnetically insulated manner from the flywheel and a signal coil disposed exterior of the flywheel and adjacent to the flush bolt and the flywheel so that magnetic flux through the flush bolt may be interlinked with the signal coil. The disadvantage of such signal source is the difficulty for the magnet and the associated pole piece to be installed on the flywheel because the flush bolt must be secured to the flywheel in a magnetically insulated relation therefrom.